It is known, that transformers are used in electrical distribution networks for coupling network sections of a first voltage level, for example 380 kV, with network sections of a second voltage level, for example 110 kV. Such high voltage transformers might have a rated power of several 100 MVA. Due to reasons of electrical insulation the whole transformer, namely including the transformer core and the transformer coils, can be arranged within a transformer vessel or tank, which is filled with oil or another insulation fluid.
It is also known, that such transformers can be equipped with a tap changer. A tap changer is an electrical switch, wherewith the transmission ratio of a transformer respectively its transformer coils can be modified. A coil with a fixed amount of windings can be electrically connected in series with a coil including several taps, whereas each tap corresponds to a certain number of turns. The tap changer connects the electrical output of such a composite coil with one of the taps and modifies the total number of active turns of the composite coil therewith.
The regulating windings of a tap-coil of power transformers can be sensitive to over-voltages excited by external transient disturbances like lightning or switching surges. For larger regulation ranges and high voltage transformers it is often difficult or even impossible to cope with the resulting voltage stresses by a proper winding design or increased electrical insulation. In such cases it is common practice to connect metal oxide varistors as surge arresters between the conductors leading from the taps of the regulating windings to the tap changer. It is also common practice to assemble a column of a specified number of varistor discs between copper contact pieces. The varistor column is kept under compression by some composite tie rods and springs. The varistor column, or several of them, is mounted inside the transformer tank, under oil, wherever there is space left near the internal electrical connections and tap changer(s).
Disadvantageously within the state of the art is the taps of the transformer coil and also the surge arresters are under oil and are therefore not accessible in a good way. Thus internal surge arresters are not accessible for inspection, replacing or testing without draining oil from the transformer tank. Also a testing or measuring of voltages of the taps is not possible without major effort.